Mana, a magical power resource used when casting, is available to all of the above classes. Hunters also use mana for their abilities, but are generally not considered casters because of their focus on physical damage, and their abilities do not scale with spell power. Death Knights can also be classified as casters, however they use runes and runic power instead of mana, and, like hunters, none of their abilities scale with spell power.

Occasionally, the term caster is used contextually in a more restrictive sense than the one offered by the dictionary definition above:

In relation to large raiddungeons, i.e. Molten Core, 'casters' often refers to players that do not need to be within melee range of enemy targets and which are reliant upon mana (and thus have specific requirements with regards to mana regeneration).

In relation to smaller dungeons, "caster" is usually used synonymously with nuker, typically mages or warlocks, since other classes with spell casting abilities would be referred to specifically as healers.

In relation to PvP, a "caster" is often synonymous with a character wearing cloth armor and relying on spells as their primary damage capacity, namely mages, warlocks and priests.

In relation to PvE, a "caster" is often synonymous with a magic-wielding mob, usually singled-out because of their low armor class, high damage potential and difficulty to tank, as in "Target the casters first."

In relation to Hunter pets, certain tameable beasts (those with a mana bar while still wild) are sometimes referred to as "caster pets". These pets have higher intellect and spirit, and lower health, armor, and DPS than most other pets — and as such are regarded by some as inferior to other pets.

Even when the context is known, "caster" can be a vague term. The only safe assumption that can be made is that it will never be used to refer to a rogue or warrior.